Consider move libarena and libgraphviz out of tree

[crlf0710]

These code doesn't get touched a lot. So maybe it makes sense to move them out instead of generating auto publish packages every day.

[matklad]

Note that libarena already exists out of tree as https://crates.io/crates/typed_arena, which is a well maintained and widely used crate.

[crlf0710]

Opened thomcc/rust-typed-arena#28 .

[Mark-Simulacrum]

I'm going to close this as I think doing this is best done as publishing a crate (perhaps a direct copy of in-tree code), getting the code in a state perhaps more fit for crates.io, and then using it in tree. Just copying the code to a separate repository does not seem helpful, and indeed, seems actively harmful.